Re: non trivial constant folding

Steve Meyer wrote:
>
> The old Bell Labs non portable C Compiler (early Unix compiler) had a
> very good heuristic method for finding foldable constants (It is
> described in Ctour document from 70s - someone posted url to it here a
> while ago but I did not save it).


Try http://plan9.bell-labs.com/7thEdMan/bswv7.html .
Download the PDF or ps.gz version of V7vol2b, and look up C tour.


> Idea is to sort expressions by
> operand complexity for communative and associative operators. Int
> constant is less complex than real, etc. Then apply constant folding
> after sorting. Then re-sort and re-fold until no progress or for a
> few times. The sorting was most complex (on left) to least complex
> order which has the added advantage that simple register assignment
> algorithms do quite well on expressions with left operands more
> complex.


Pretty much. This part wasn't explicitly iterative--it was done once,
though I suppose the recursiveness and other accidents of coding
got the routines called more than once on occasion.


Dennis

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